This invention relates to treating articles in a plasma glow discharge and, in particular, to etching semiconductor wafers in a magnetically confined plasma at low pressure.
In the prior art, a variety of processes have evolved for treating semiconductor wafers. For etch processes, measures of quality have included uniformity and etch rate. In the quest for improving quality, and partially as a result of treating larger wafers than previously treated, progressively more RF power is applied to the plasma. A difficulty arises with increasing the power due to the decrease in the size of the devices being formed on the wafer. The devices are more delicate and some devices cannot withstand the applied power.
One solution to this has been to generate the plasma remotely, e.g. using microwave radiation, and locating the wafer downstream from the generator. This approach is not without its own difficulties.
Another approach has been to magnetically confine the plasma. This has the effect of electrically removing the conductive walls of the chamber from the plasma by reflecting ions or electrons back into the interior of the discharge. The result is a more efficient generation of ions with reduced power.
This solution is incomplete or, more accurately, introduces its own problem. Minimum device damage depends on generating ions having the least energy possible yet still effective in etching. In general, this occurs at relatively low pressures, lower than commonly used previously, e.g. one millitorr. At such pressures, commercially available manometers, even high quality ones, become inaccurate. For example, a manometer exposed to atmospheric pressure typically has an offset error of several millitorr. Thus, measuring pressure at one millitorr or below becomes an estimate at best.
In view of the foregoing, it is therefore an object of the present invention to provide improved means for measuring or controlling gas pressure in the range of sixty millitorr and below in magnetically confined glow discharges.
In accordance with another aspect of the present invention, it is an object to provide means for optimizing a plasma etch cycle.